<!doctype html>
<style>
  body { margin: 0 }
  :root {
    font-size: 10px;
  }
  .container {
    font-size: 20px;
  }
  line {
    stroke-width: 40px;
    stroke: lime;
  }
  svg {
    background-color: red;
  }
</style>
<div class="container">
  <svg width=400 height=400>
    <rect width=400 height=400 fill="blue"/>
    <line y1=0   y2=0   x1=0 x2=200   />
    <line y1=40  y2=40  x1=0 x2=10em  />
    <line y1=80  y2=80  x1=0 x2=20rem />
    <line y1=120 y2=120 x1=0 x2=50%   />
    <line y1=160 y2=160 x1=0 x2=4vw   />
  </svg>
</div>
